The ways in which the functional properties of nerve cells controlled over long periods (days, weeks, or months) are largely unknown. That neuronal properties do change on this time scale is clear from the remarkable abilities of higher nervous systems. At the level of individual neurons, three areas of investigation appear to hold special promise at present for understanding how nerve cell properties are modulated. These are 1) the effects on neuronal properties of the presence or absence of contact with the 'target organ' and the mechanism of these effects, 2) the effects of innervation (or lack of it) on a neuron's properties, and 3) the ability of a neuron to 'select' between functionally appropriate and inappropriate inputs. The purpose of this work is to explore these three areas by applying the techniques of intracellular recording and quantitative electron microscopy to neurons of the mammalian peripheral autonomic nervous system. The ultimate aim is to define the mechanisms by which long-term changes normally come about in the mammalian nervous system.